1. Field of the Invention
This invention relates to an information storage medium capable of reading out or recording information by irradiating energy beams, and an information playback method and an information recording method each using the medium.
2. Description of the Related Art
To improve a storage capacity in an ROM type optical disk, there has been proposed that a plurality of marks are arranged inside a spot and that a readout signal level is changed to multi-levels in accordance with the number of marks. For example, literature “Two-Dimensional Optical Storage”, Optical Data Storage 2003, Technical Digest TuB1 pages 90-92, discloses a structure in which one pit having a perfect circle shape is disposed at a center and six pits are arranged around the center pit. According to the structure, user data is converted to the number of marks and is recorded. At a time of the readout, the number of marks is detected by use of a correspondence relation between the number of marks and reflected light intensity level, and is then returned to the original user data.
JP-A-8-031015 discloses a technology that uses a plurality of phase pits having different depths, provides an optical phase difference between the adjacent pits, and linearizes non-linearity of readout signal levels.
In the ROM described above, the pit interval becomes remarkably narrower in comparison with that in the conventional two-level recording system. Therefore, interference between the pits becomes remarkably strong. When the number of marks is increased, a change of quantity in one pit is not certain, and when the number of marks is great, the change of quantity becomes zero. In other words, the relation between the number of pits and the readout signal level is not linear. When linearity is lost, it becomes difficult to detect the number of pit from the readout signal level, and a detection error increases at a point at which the change of quantity in one pit is small.
JP-A-8-031015 also discloses the phase pit depths in the combinations of (π/2, 3π/2), (4π/3, 8π/3) and (2π/3, 4π/3), but cannot cope with high density recording.